1. Field of the Invention
This invention relates to pressure exchangers for transfer of energy from one liquid flow to another. More specifically, this invention relates to pressure exchangers for the transfer of energy from one liquid stream to another using a rotating rotor.
2. Summary of the Prior Art
The present invention provides a device which can be appropriately described as an engine for exchanging pressure energy between relatively high and relatively low pressure fluid systems, which the term fluid being defined here as including gases, liquids and pumpable mixtures of liquids and solids. The engine for pressure energy exchange of the present invention is a highly efficient device with well over 90% of the energy of pressurization in a pressurized fluid system being transferred to a fluid system at a lower pressure. The device employed for achieving this highly efficient transfer has a long and trouble free operating life which is not interrupted by the plugging and fouling of valves, or the binding or freezing of sliding pistons or the like.
In processes where a liquid is made to flow under pressure, only a relatively small amount (about 20%) of the total energy input is consumed in pressurizing the liquid, the bulk of the energy being used instead to maintain the fluid in flow under pressure. For this reason, continuous flow operation requires much greater energy consumption than non-flow pressurization.
In some industrial processes, elevated pressures are required only in certain parts of the operation to achieve the desired results, following which the pressurized fluid is depressurized. In other processes, some fluids used in the process are available at high pressures and others at low pressures, and it is desirable to exchange pressure energy between these two fluids. As a result, in some applications, great improvement in economy can be realized if pressure exchange can be efficiently transferred between two liquids or between pumpable slurries of liquid-solid mixtures.
By way of illustration, a specific process of this type is the exchange crystallization process for effecting desalination of sea water, or other saline aqueous solutions. In this process, a slurry of ice and an exchange liquid, such as a hydrocarbon, is placed under extreme pressure in order to reverse the order of freezing so that the ice crystals melt, and the exchange liquid is partially frozen. Following this step of the desalinization process, the water from the melting of the ice is separated from the hydrocarbon, which is in the form of a slurry of solid hydrocarbon particles with the liquid hydrocarbon, and the separated phases are then depressurized to near atmospheric pressure. The economy with which the exchange crystallization desalination process can be practiced is directly dependent upon the efficiency with which the energy input to the process upon pressurization of the ice-exchange liquid system can be recovered after separation of the water-exchange liquid phases.
Another example where a pressure exchange engine finds application is in the production of potable water using the reverse osmosis membrane process. In this process, a feed saline solution is pumped into a membrane array at high pressure. The input saline solution is then divided by the membrane array into super saline solution (brine) at high pressure and potable water at low pressure. While the high pressure brine is no longer useful in this process as a fluid, the pressure energy that it contains has high value. A pressure exchange engine is employed to recover the pressure energy in the brine and transfer it to feed saline solution. After transfer of the pressure energy in the brine flow, the brine is expelled at low pressure to drain.
Accordingly, pressure exchangers of varying design are well known in the art. U.S. Pat. No 3,431,747 to Hashemi et al. teaches a pressure exchanger for transfer of pressure energy from a liquid flow of one liquid system to a liquid flow of another liquid system. This pressure exchanger comprises a housing with an inlet and outlet duct for each liquid flow, and a cylindrical rotor arranged in the housing and adapted to rotate about its longitudinal axis. The cylindrical rotor is provided with a number of passages or bores extending parallel to the longitudinal axis and having an opening at each end.
A ball is inserted into each bore for separation of the liquid systems. The ball movement is limited due to the use of a seat at each end of the passages. The seats cause a reduction in cross-area of the bores and are susceptible to wear and eventual failure. A more significant problem with this invention however, is that the bores of the cylindrical rotor line up with respective outlet ports for a very limited time. In this arrangement, fluid flow is not continuous, but is rather shut off and on as the cylindrical rotor spins. This results in very low efficiency as well as increased mechanical wear of the various parts due to pressure transients in the system.
In an attempt to improve the overall efficiency of this type of pressure exchanger, a modified pressure exchanger for liquids can be found in U.S. Pat. No. 4,887,942 to Hauge. Similar to the pressure exchanger found in Hashemi, a cylindrical rotor is spun inside a housing for the communication of pressure energy between a low and high pressure liquid source. Located in the rotor is an array of longitudinally running passages for the communication of the flowing liquid to inlet and outlet ports. The inlet and outlet ports of the Hauge pressure exchanger however is comprised of two semi-circular shaped ducts that allow for the almost continuous flow of liquid from the passages to the ducts. Allowing for the almost continuous, uninterrupted flow of liquid increases the pressure exchanger efficiency as well as reduces wear and tear on the mechanical components connected to the device.
Referring to FIG. 1 which shows a cross-sectional view of the Hauge invention, a major drawback of the Hauge invention is the reduction in sealing surface-area between the inlet and outlet ports. The two semi-circular ducts are separated by a very thin wall, thereby requiring extremely tight fitting components to ensure an acceptable level of sealing and the prevention of pressure loss between the high and low pressure ports. Leakage between these two ports results in reduced efficiency of the pressure exchanger, and as the tight tolerances of the mechanical components begin to wear, leakage between the ports will only increase and require costly maintenance.
There therefore is a need for a pressure exchanger which provides both smooth and uninterrupted fluid exchange as well as enhanced sealing capability thereby reducing the amount of leakage that occurs between the high and low pressure ports.
The primary objective of the present invention is to provide a device for efficiently transferring the energy of pressurization from a pressurized fluid to a second fluid at a lower pressure.
Another object of the present invention is to provide a device for efficiently transferring the energy of pressurization from a pressurized fluid to a second fluid at a lower pressure which exhibits enhanced sealing properties between the two pressurized fluids.
Yet another object of the present invention is to provide a pressure exchanger that allows for an almost continuous flow of fluids thereby increasing overall efficiency as well as reducing deleterious transients within the pressure exchanger.
Still another object of the present invention is to provide a pressure exchanger that has reduced maintenance costs and an increased usable life.
Yet another object of the present invention is to provide a device that allows for the exchange of pressure energy between two fluids with the use of conventional in line valving.
In addition to the described objects and advantages of the present invention, additional objects and advantages will become apparent as the following detailed description of the invention is read in conjunction with the accompanying drawings.